1. Field of the Invention
This invention relates to an optoelectronic keypad for a measuring instrument, including a transparent touch screen, at least two touch-sensitive control buttons, at least two light emitters and at least two light receivers i.e. photodetectors, where said control buttons are located next to each other on one side of the touch-screen surface, the light emitters and the photodetectors are situated next to each other on the other side of the touch screen and each one control button, light emitter and photodetector are functionally associated with one another so as to constitute one action key, whereby pressing the control button of an action key causes the light emanating from the light emitter of that action key to be reflected at the actuated key and onto the photodetector of that key. The invention further relates to a method for controlling this type of optoelectronic keypad for a measuring instrument. In this context, the above statement whereby the light emanating from the light emitter of the action key is reflected at that key when actuated means more specifically that the light is reflected for instance off a finger that is brought close to the key or presses the control button.
2. Description of the Prior Art
An optoelectronic keypad of the above type is described for instance in EP 0 618 680 A1. That particular optoelectronic keypad encompasses infrared light emitters and infrared photodetectors, with functionally associated infrared light emitters and receivers so positioned relative to each other that, when for instance a finger touches the respective control button or is close to it, the light from the infrared light emitter is retroreflected in such fashion that it impinges more or less exclusively on the infrared photodetector paired with the infrared light emitter concerned. In addition to the necessary alignment of the infrared light emitter with the corresponding infrared photodetector, infrared-reflective baffles and, respectively, non-infrared-reflective covers are provided. These are intended to prevent the light reflected off a finger on a given button, when touched, from being detected by infrared photodetectors other than the infrared photodetector assigned to the infrared light emitter whose light was reflected by the finger. If the retroreflected light were picked up by more than one infrared photodetector, it would no longer be possible to unambiguously determine which key was in fact actuated, or whether it was one key or two keys that were pressed at the same time.
It follows that in the optoelectronic keypad according to EP 0 618 680 A1, the limiting factor for the shortest possible distance between two keys is determined by the requirement that light emanating from the infrared emitter of a given key and reflected by the finger that is in contact with the control button of that key must not be picked up by the infrared receivers of neighboring keys. Another limiting factor for the smallest possible distance between two neighboring keys consists in the fact that, in the event of high reflectivity, even a slight, partial contact with a key, for instance when a finger touches not only the button surface of one single key, can cause several keys to respond.
Other problems associated with prior-art optoelectronic keyboards and keypads include the fact that intense ambient light such as bright sunshine can make it difficult to tell whether a key was pressed because the bright incident light has overpowered the photodetectors. Equally difficult, in the case of poor reflectivity, is the determination of whether a key was actuated, for instance when a key is pressed with a black glove covering the fingers. And finally, in practical implementation it is difficult to meet the requirement whereby the light detected by a receiver should essentially be limited to that reflected by the finger on the actuated key and not that transmitted by the touch screen, least of all by the outside surface of the latter, making it necessary to keep the distance between the light emitter and photodetector and the touch screen within very precise tolerances. Even minor spacing deviations, causing multiple reflections, lead to detection errors.